1. Field of the Invention
The present invention relates to pattern inspection of a photomask used in the process of manufacturing semiconductor devices, display devices such as a PDP (Plasma Display Panel) and LCD (Liquid Crystal Display), and magnetic devices such as a magnetic head of a hard disk.
2. Description of the Related Art
As element structure of a semiconductor device becomes finer, Optical Proximity Effects (OPE) by which a mask pattern formed on a photomask (reticle) used in manufacturing process is not transferred onto a wafer as designed data have been actualized in recent years. Therefore, the technology called Optical Proximity Corrects (OPC) is applied in forming the mask pattern. According to the Optical Proximity Corrects, a correction pattern with the influence of the Optical Proximity Effects taken into account is added to the mask pattern in advance, to thereby make the pattern formed on the wafer the same as the design data.
As conventional inspection methods of the mask pattern, an inspection method of detecting a mismatched part by comparing the mask patterns with each other (die-to-die comparison method), and an inspection method of detecting a mismatched part by comparing the mask pattern with the design data (die-to-database comparison method) are used. The quality of the mask pattern is decided whether or not desired device characteristics are realized by the pattern formed on the wafer by the pattern transfer. Therefore, a defect is determined by comparing the measure of the mismatched part with allowable measure for every pattern shape and area. Incidentally, the allowable measure is set so that the desired device characteristics are realized by the pattern formed on the wafer by the pattern transfer. Namely, when the desired device characteristics are realized by the pattern formed on the wafer by the pattern transfer, the detected mismatched part is determined to be a false defect, not the defect.
However, a pattern of a fine dimension which is determined to be the false defect by the conventional inspection method of the mask pattern has come to be corrected according to the Optical Proximity Corrects. Therefore, it is impossible to accurately determine the defect in the fined pattern according to the conventional inspection method of the mask pattern.
A pattern inspection method of the reticle to solve the above problem is disclosed in, for example, Japanese Patent Application Publication No. Hei 9-297109 and so on. According to this pattern inspection method, light intensity distribution of a reticle pattern to be an inspection object and light intensity distribution of a design pattern are found by simulation. It should be noted that the light intensity distribution obtained by the simulation corresponds to light intensity distribution which is actually obtained through a reduction optical system of a stepper. Then, the reticle pattern is inspected by comparing the light intensity distribution with one another.
According to this pattern inspection method, the patterns (light intensity distribution) which are formed on the wafer by the pattern transfer using the reticle pattern and the design pattern are compared with each other, and therefore it is possible to determine with high accuracy the presence/absence of the defect that matters with respect to the characteristics of the device formed on the wafer. However, since the reticle pattern and the pattern formed on the wafer does not agree with each other, defect location in the reticle pattern cannot be identified even though defect location in the pattern formed on the wafer can be identified by comparing the light intensity distribution. Therefore, in order to correct the defect in the reticle pattern accurately, defect correction and inspection of the reticle pattern need to be repeated depending on experiences and intuitions of an operator. Namely, it is impossible to correct the defect in the reticle pattern with ease. As a result of this, production cost of the reticle increases. In other words, manufacturing cost of a semiconductor device increases. Therefore, an inspection method which can determine the presence/absence of the defect in the reticle pattern with high accuracy and identify the defect location in the reticle pattern is required.
An appearance inspection method of a printed circuit board on which components are mounted is disclosed in, for example, Japanese Patent Application Publication No. 2002-243426 and so on. According to this appearance inspection method, the image of an inspection object and the image of a reference object are first subjected to a Fourier transform, respectively, to obtain optical diffraction patterns. Next, the obtained optical diffraction patterns are superposed on each other, to optically find a difference or perform Exclusive OR. Then, the result of the difference or Exclusive OR is subjected to an inverse Fourier transform, to obtain the image of a defect part in the inspection object. Thus, component absence, component fitting abnormality and the like of the printed circuit board can be detected.
According to this appearance inspection method, whether or not the shape of the inspection object agrees with the shape of the reference object is inspected. Namely, whether or not the component in question is mounted on the printed circuit board as expected is inspected. On the contrary, according to the pattern inspection of the photomask, whether or not the desired device characteristics are realized by the pattern formed on the wafer by the pattern transfer needs to be inspected.